In the old days, you took the thermostat out and put it in a boiling container to see if it open and closed, if it did that then you knew it still worked. Today with some thermostats, if you do that, you will destroy it as there is a ball of wax which will melt leaving the the thermostat permanently open or closed - can't remember which.
I had "blow back" in my Mazda Protégé after 330,000 Kilometres and after replacing the water pump and the radiator, the mechanic looks at me and says, "oh yeah, if you have blow back, its your head gasket and rings giving you this problem. Don't do anything else until you get a new ring job (I bought a 528i instead)." He gave me the new thermostat he was going to install and said not to install it until after the ring job, if I did, the wax inside would melt when the car heated up too much and I would kill the thermostat - so I drove the car for 6 months without a thermostat.